The techniques known as “jet grouting” are used to form columnar structures of artificial conglomerate in the ground. These techniques are based on the mixing of particles of the soil itself with binders, usually cement mixtures, which are injected at high pressures through generally small radial nozzles formed in an injection head (commonly referred to as a “monitor”), fixed in the proximity of the lower end of a string of tubular rods which is rotated and withdrawn towards the surface. At the bottom of the string of rods, under the monitor, there is fixed a drilling tool which is lubricated, during the excavation phase, with a drilling fluid supplied through the rods, which, in this case, act as ducts.
The jets of binder are dispersed and are mixed with the surrounding soil, thus creating a conglomerate block, generally of cylindrical shape, which, when hardened, forms a consolidated area of soil.
The strings which are presently most commonly used in the foundations sector have a duct with a large cross-section through which the mixture of water and cement is supplied to the monitor zone, where the nozzles are present. The latter are housed in radially oriented holes, i.e. perpendicular to the longitudinal axis of the monitor. In terms of fluid dynamics, this configuration reduces the friction losses along the path, since the flow velocity of the fluid is low so long as the fluid does not reach the end of the monitor. Once the fluid has reached this zone, the stream deviates orthogonally in the region of the nozzle, also creating irregular free motions characterised by strong turbulence in the region in which the stream deviates. This brings about a high head loss, right in the proximity of the outlet from the nozzles, as a result of turbulence which prevents the stream from exiting the nozzles in an ordered manner, i.e. with the velocity vector of the single particle of material exiting oriented according to the main axis of each nozzle.
The procedures by which the fluid passes from the inside to the outside of the monitor are the cause of considerable head losses and are therefore understood not just in terms of increased power consumption but also in terms of a reduced diameter of the column of treated material. There is thus a need in the field to limit the head losses generated within the monitor.
The patent literature discloses various monitors for the jet grouting sector which, in their interior, have a plurality of channels that are twisted according to a layout with multi-helical geometry and are able to guide the stream in a helical motion from the inlet of the monitor to the inlet of the relative nozzle. One example is given by JP-A-2008285811. This type of multi-helical geometry does not guarantee per se the maximum improvement in performances with respect to the conformation usually used (i.e. that which generates a turbulent free motion), unless the fundamental parameters for the correct dimensioning of said structure are identified and the inlet and outlet zones of the jet are modified so as to maximise efficiency.
The patent literature also describes other monitors having one or more curved ducts for deviating the fluid mixture, conveying it from the main duct towards the side nozzles, following paths with gradual changes in direction, thereby reducing the turbulences and the concentrated head losses. U.S. Pat. No. 5,228,809 discloses a duct with a constant cross-section and regular curvature. EP-1396585 discloses progressively tapered, variable curvature ducts. However, the diameter of the ducts for the passage of the fluid mixture along the entire final inlet length to the nozzles is conditional on the need to balance two opposing requirements: firstly, it is necessary to limit the external dimensions of the monitor (generally relatively small and of the order of magnitude of about 100 mm); secondly, it is desirable to give the ducts the best radius of curvature possible. In other words, these systems provide a length which has an appreciable length and a reduced diameter and is comparable to that of the outlet for the nozzle. Therefore, the advantage derived from the reduced concentrated losses is limited by the fact that the fluid adopts a very high velocity within the final length, with very high resulting friction losses. In addition, the presence of ducts, curves and radiuses greatly complicates the overall architecture of the monitor, making the assembly, maintenance and disassembly steps much more complex.